I have a question , what is the most reliable server OS to use for Internet,

I have to say FreeBSD or OpenBSD for that one.

Extremely robust and reliable OS for about any kind of servers (web, mail, file, db, shell, firewall, IDS, etc) and quite easy to setup when you know your way around a UNIX machine.
Very secure, quite barebone, good package manager (pkg_add == awesomeness), easy to maintain, easy to backup lots of features (like the 'jails' for FreeBSD), very well documented, lightweight, you have total control over the OS and stable as heck.

Personally I would -NEVER- touch a Windows machine for a web server unless it's behind a barricade (as in exetremely good firewall + IDS) and hidden deeply in a DMZ, especially if it's to contain important data. I mean if it's a game server no one will really try to screw it up and if someone does, reinstall (from disk or backup) and you are done.

Also I have to say that unless you don't know your way around a terminal and/or a UNIX machine, I wouldn't touch anything based off Red Hat. I know it's an industry standard and there's a crap load of machine running it, but again Windows is supposed to be one too and it's not making it any better. If you catch my drift, it's far from being the best OS around for web servers unless you -need- a mouse to start the daemons ("services") and you can't either start them from the terminal or make your own scripts or screw around the configuration files.

For servers you have to tell yourself one thing, ease of use comes at a price... instability and insecurity.

I would have said for the OS recommendation NetBSD since it's my favorite OS by far, but it's quite a pain to setup for somebody who doesn't know very well UNIX system but omfg it is all the quality of the second paragraph + tweakable and modifiable.

__________________
"NIX is a classic example of security through obscurity because there is no real monetary reward for crackers and hackers to break Linix" -AkG